1. Field of the Invention
This invention relates to a resin packaged semiconductor device and, more particularly, to a resin insulated type semiconductor device having a thin resin film for electric insulation and heat conduction formed on the rear surface of a frame on which a semiconductor chip is mounted.
2. Description of the Related Art
A resin insulated type semiconductor device can be constructed as shown in FIGS. 5A and 5B or FIGS. 6A to 6C.
In semiconductor device 50 shown in FIGS. 5A and 5B, 51 denotes a semiconductor chip; 52, a chip mounting section (island) of a lead frame; 53, an external connection lead; 54, a lead wire for electrically connecting semiconductor chip 51 to outer connection lead 53; 55, a cap; 56, a moulded resin for an outer casing; 57, a mounting hole formed in part of the outer casing for fixing the semiconductor device; and 58, a frame fixing section formed at the tip end portion of the lead frame to extend out of the outer casing and used for fixing the lead frame in position when the lead frame is resin-sealed.
In semiconductor device 50, resin film 59 having thin and constant film thickness is formed on the rear surface side of island 52 by resin-sealing the load frame. When this resin-seal is performed, the tip end portion (frame fixing section 58) of the lead frame and the base end portion (outer connection lead 53) are fixed.
In semiconductor device 60 shown in FIGS. 6A to 6C, the lead frame is resin-sealed. When this resin-seal is performed, the front and rear surfaces of the lead frame are held by frame supporting pins of a metallic mould. The semiconductor device is similar to that of FIGS. 5A and 5B, except that the tip end portion of the lead frame does not exposed to the outer casing, and that frame supporting holes 61 and 62 are formed in the front and rear surfaces of the casing in position corresponding to the supporting portions of the lead frame. Portions in the semiconductor device of FIGS. 6A to 6C, which are the same as those in that of FIGS. 5A and 5B, are denoted by the same reference numerals.
In a case where semiconductor device 50 of FIGS. 5A and 5B is actually used, electrically conductive extraneous substances such as metal scrapings caused by use of a tap screw and/or wet dust may come into a gap between frame fixing section 58 and a metal member (for example, a chassis of electronic instruments or a heat radiation plate to which semiconductor device 50 is mounted) provided near the lead frame. In such a case, the dielectric strength of a portion between frame fixing section 58 and the metal member will be lowered. Further, since frame fixing section 58 is formed to extend out of the outer casing, an accident such as an electric shock may occur in the practical use.
In semiconductor device of FIGS. 6A to 6C, frame supporting holes 61 and 62 are formed in the front and rear surfaces of the outer causing, and the lead frame is exposed at the internal portion (bottom portion) of frame supporting holes 61 and 62. In the practical use, part of the lead frame, which is exposed at the internal portion of frame supporting hole 62 on the rear surface side, faces a metal member (heat radiation plate or chassis) provided near the lead frame. The dielectric strength of the rear surface of the semiconductor device is determined by the dielectric strength of the facing portion. If metal scrapings are entered into the facing portion, the dielectric strength of the rear surface of the semiconductor device is lowered.
In order to solve the problem described above, it may be considered that the exposed portion of the lead frame (such as the tip end portion of the frame and the exposed frame in the frame supporting hole) may be coated with insulative resin (for example, epoxy resin or silicone resin) after the resin-sealing operation. However, in this case, it is difficult to perform the desired coating operation because of limitations due to the size of the outer causing and the like. Thus, it is almost impossible to sufficiently enhance the dielectric strength even if the insulative film is coated. Further, the insulative film could be accidentally peeled off and thus the reliability thereof is not sufficiently high.
As described above, the semiconductor device having the frame supporting holes formed in the front and rear surfaces of the outer casing is mounted on the heat radiation plate or the like. At this time, the dielectric strength of the semiconductor device is determined by the exposed portion of the frame inside the frame supporting holes and the mounting heat radiation plate which faces the exposed portion. In this case, if conductive foreign material is introduced into the facing portion, the dielectric strength is lowered.